An equatorium (plural, equatoria) is an astronomical calculating instrument. It can be used for finding the positions of the Moon, Sun, and planets without calculation, using a geometrical model to represent the position of a given celestial body.
The earliest extant record of a solar equatorium, that is, one to find the position of the sun, is found in Proclus's fifth century work Hypostasis,[1] where he gives instructions on how to construct one in wood or bronze.[2] Although planetary equatoria were also probably made by the ancient Greeks,[2] the first surviving description of one is from the Libros del saber de astronomia (Books of the knowledge of astronomy), a Castilian compilation of astronomical works collected under the patronage of Alfonso X of Castile in the thirteenth century, which includes translations of two eleventh century Arabic texts on equatoria by Ibn al‐Samḥ and al-Zarqālī.[2] Theorica Planetarum (c. 1261-1264) by Campanus of Novara describes the construction of an equatorium, the earliest known description in Latin Europe.[3] Richard of Wallingford (1292–1336) is known to have built a sophisticated equatorium named Albion that could calculate lunar, solar and planetary longitudes. Unlike most equatoria, the Albion could also predict eclipses.[4]
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